Bovine factor Xa will cleave factor VIII in at least three regions resulting in factor VIII that expresses as much as two-fold increased activity in a one-stage clotting assay. Very small amounts of fXa can be used such that the rate of cleavage is slow and can be followed for up to 60 minutes. The cleavage can also be demonstrated by immunoblot on which the disappearance of some fVIII bands and the appearance of the corresponding cleavage fragments can be shown. Tissue factor pathway inhibitor (TEPI) is known to bind and inhibit fXa. This complex then binds to tissue factor/factor VIIa and inhibits the formation of more Xa. We are studying the ability of TFPI to also inhibit the Xa-induced cleavage of factor VIII.